Return values of operators
Quoting from the API documentation

Operators don’t have return values as you might expect, instead they return a set() which is made up of: {‘RUNNING_MODAL’, ‘CANCELLED’, ‘FINISHED’, ‘PASS_THROUGH’}. Common return values are {‘FINISHED’} and {‘CANCELLED’}.

You might get the arguments passed to a registered operator like this:
bpy.context.active_operator.properties['keyword']

or
bpy.context.window_manager.operators[-1].properties['keyword']

For more information about linking see also BlendDataLibraries and Library.